1. Technical Field
The present invention relates to a device for optically reading information codes placed on a subject.
2. Related Art
A code reader apparatus is conventionally known that optically reads one-dimensional codes, such as barcodes, and two-dimensional codes, such as QR codes (registered trademark). Extending the range of distances at which read-out can be performed (referred to, hereinafter, as a read-out range) of the code reader apparatus leads to significant improvement in product value of the code reader apparatus. To extend the read-out range, a technology is required for extending depth of field. The depth of field refers to the range of distances on the field side at which the field appears in a focused state.
JP-B-3275010 discloses “an optical system having an extended depth of field”. The optical system includes a phase plate optical system and restored image processing performed in frequency space. In the optical system disclosed in JP-B-3275010, the depth of field is extended. An image that is in focus can be acquired over a wide range of distances. However, the code read-out range cannot be extended to its maximum limit by merely extending the depth of field as in the optical system described in JP-B-3275010.
Here, pixel assignment in instances in which codes of the same size are imaged is considered. At near field, the code is projected onto the overall light-receiving surface of an image sensor. However, at far field, the code is projected onto a portion of the light-receiving surface. The number of pixels assigned to a single cell of the code is fewer at far field, compared to that at near field. In other words, a higher resolution is required at far field, compared to near field.
For example, when read-out range, code size, and cell size are determined, an angle of view of an imaging system is determined from the code size at minimum distance. The required resolution is determined from the pixel assignment at maximum distance. A relationship is established in which the number of pixels required in the image sensor is determined from the angle of view and resolution. In actuality, the number of pixels of the image sensor is often determined in advance from the perspective of cost. The maximum distance is often determined by the resolution. Therefore, if the depth of field is merely extended, the read-out distance at near field is restricted by reading screen limitations, such as the code exceeding beyond the reading screen, i.e., the image sensor surface.
In addition, a product catalog for a handheld cordless scanner (product name “SR61ex”) manufactured by Intermec Inc. discloses a “code reader” that uses an auto-focus lens to extend the code read-out range. In the method using the auto-focus lens, the distance between the lens and the light-receiving surface of the image sensor is changed by the lens being moved. A code reader that uses the auto-focus lens method requires a driving mechanism for moving the lens. In addition, JP-A-2012-515359 discloses a “code reader” that extends the code read-out range using a variable-focus lens. In the method using the variable-focus lens, the focal distance of the lens is changed. A code reader that uses the variable-focus lens method requires a mechanism for changing the focal distance of the lens.